A head-mounted display (HMD) system is a type of wearable device with increasing popularity within the consumer electronics industry. HMDs, along with similar devices such as helmet-mounted displays, smart glasses, and virtual reality headsets, allow users to wear a display device such that the hardware remains fixed to their heads regardless of the user's movement.
When combined with environmental sensors such as cameras, accelerometers, gyroscopes, compasses, and light meters, HMDs can provide users with experiences in virtual reality and augmented reality. Virtual reality (VR) allows a user to be completely submerged into a virtual world where everything the user sees comes from the display device. Devices that provide augmented reality (AR) allow users to optically see the environment, and images generated by the display device are added to the scene and may blend in with the environment. Accordingly, traditional VR and AR technology involve a display that is mounted in front of the user's head with a lens configuration that determines the virtual image position and field.
A basic layout of typical commercial VR or AR systems (both involving and not involving use of a smartphone for a display) includes a display device and a lens structure that images the display light into the far field to enable comfortable viewing. To ensure sufficient magnification, with wide field of view and to have a virtual image at a far enough distance from the eye, the size of this arrangement is restricted. In addition, the display is a relatively far distance from the eyes, meaning that the device must be strapped to the head to not fall off. Furthermore, the weight of the device is far forward when worn, meaning that long term viewing could become tiresome on the face and neck due to the torque generated about the head by the weight of the device. Lens elements used in such systems may be configured as a normal curved surface lens of known type, or a structured Fresnel lens with angled features of known type, or other known lens arrangements involving one or more lenses.
One of the primary elements of HMDs is the display module mounted onto the head. However, since the unaided human eye cannot accommodate (that is, change optical power to provide a focused image) for images closer than a certain distance from the eye, eyepiece lenses are required to re-image the display module such that the display image appears to be at a comfortable viewing distance from the user. Such an optical configuration requires substantial space between the eyepiece and the display module. Furthermore, complex lenses are needed if the HMD needs to display images with high quality and a wide field of view (FOV). The result of these requirements in conventional systems is a heavy and bulky headset that is uncomfortable to wear for any length of time, and the size is limited by basic optics to achieve the correct magnification and the virtual image distance.
The distance separating a person's eyes is known in the art as the interpupillary distance (IPD), which for adults tends to range from around 50 mm to 70 mm. For optimal image quality, a corresponding IPD of the eyepiece lens components of an HMD system should match the IPD of the user. A typical HMD that uses fixed optics will show aberrations if the user's IPD is significantly different from the IPD specified by the HMD's optical design. To minimize these aberrations, an HMD may have optics that can be physically adjusted to accommodate the specific IPD of the user.
There are a number of conventional methods that are commonly used to adjust the IPD for HMDs. For example, many commercial HMDs employ a physical mechanism that allows the user to change the distance separating the eyepiece lenses. For example, CN 206805007 (filed Dec. 26, 2017) describes a method for adjusting interpupillary distance of an HMD using a scissor-type connector and screw. CN107015363 (Huang, published Aug. 4, 2017) describes a method for adjusting interpupillary distance using a rack and pinion mechanism. The IPD value used to render images onto the display is then changed either in software to a set value, or automatically though a sensing system that senses the positioning of the system eyepieces. The image displayed on the display panel is then adjusted accordingly. For example, WO 2014163869 (Stafford, published Oct. 9, 2014) describes a method for adjusting interpupillary distance in an HMD and subsequently updating the images displayed. In all these systems, the display panel remains fixed within the HMD and the optical components of the eyepieces are moved relative to the display panel.
There have been other attempts to reduce HMD form factor using multiple small lenses with overlapping images that modify the magnification required. For example, Applicant's commonly owned Application GB 1621621.0, filed Dec. 19, 2016, describes an alternative image overlap system with two displays and a folded W-shaped mirror arrangement. The field of view in this case is defined by the maximum aperture and path length. Although such a system reduces the overall form factor during use as compared to prior conventional configurations, in this initial design a mechanism of adjusting the IPD optimized for such a system is not described.